This invention relates to laser mirrors for a helium-neon (He-Ne) laser tube, and particularly to structures of laser mirrors each having a multilayer dielectric coating structure with an effect to increase laser output at wavelength .lambda..sub.0 =6328 .ANG. and reduce laser output at wavelength .lambda.=3.39 .mu.m.
A He-Ne laser tube produces laser oscillation at .lambda..sub.0 =6328 .ANG. on the corresponding transition from level 3S.sub.2 to level 2P.sub.4, and also at .lambda.=3.39 .mu.m on the corresponding transition from level 3S.sub.2 (common level with the above-mentioned) to level 3P.sub.4. Owing to the by far greater amplification gain of the oscillation at .lambda.=3.39 .mu.m than that at .lambda..sub.0 =6328 .ANG., often both oscillations take place simultaneously in the same laser tube. In other words, the so-called competitive phenomenon is found. In this case, therefore, the output becomes decreased compared with a single oscillation at .lambda..sub.0 =6328 .ANG. alone. This phenomenon is not so remarkable at small power outputs of 1 mW order but becomes marked particularly for high power output He-Ne laser apparatus of 5 mW or more.
As a proposal for solving this difficulty, is disclosed in Japanese Patent Publication No. 63-26560 an example of He-Ne laser mirror designed to have such a spectral reflectivity that reflectivity at .lambda..sub.0 =6328 .ANG. is great and reflectivity at .lambda.=3.39 .mu.m is small. This Japanese Patent publication additionally teaches that the multilayer dielectric coating structure of the output mirror should be composed of a minimized number of layers, and that for allowing a part of laser beam at .lambda..sub.0 =6328 .ANG. oscillating in the He-Ne laser tube to escape from there, the output mirror is required to be generally about 1.1% to 1.9% in transmittance at .lambda..sub.0 =6328 .ANG., and correspondingly about 98.1% to 98.9% in reflectivity. The multilayer coating on a transparent substrate of glass is composed of high refractive index layers (H) of titanium dioxide and low refractive index layers (L) of silicon dioxide, stacked alternately H and L. Of the coating layers, only the top has an optical thickness of .lambda..sub.0 /2(.lambda..sub. 0=6328 .ANG.), and each of the other layers has a thickness of .lambda. .sub.0 /4. Thus the structure of the output mirror is represented as "Sub. .vertline.H.multidot.L).sub.x -H.multidot.2L.vertline. Air". The term "Sub." means a substrate. According to this Japanese Patent Application, no structure can meet the condition for use as the output mirror where transmittance at the .lambda..sub.0 =6428 .ANG.is within a range of 1.1 to 1.9%, except x=5 in the formula, that is, 12 layers. This structure, however, gives a great reflectivity at 3.39 .mu.m as much as 7.5%. For overcoming this drawback, has been proposed another 12-layer structure represented as "Sub. .vertline.(H.multidot.L).sub.5 -Z.multidot.2L.vertline. Air", wherein Z is a dielectric layer of zirconium oxide or lanthanum oxide, of which the optical layer thickness is .lambda..sub.0 /4 and the index of refraction is between H and L.
On the other hand, it has been proposed for the total reflection mirror to have a 20-layer coating, i.e. structure expressed as "Sub. .vertline.(H.multidot.L).sub.9 -H.multidot.2L.vertline.Air" or 22-layer coating, i.e. structure expressed as "Sub. .vertline.(H.multidot.L).sub.10 -H.multidot.2L.vertline.Air".
The above-mentioned structures of the output mirror and the total reflection mirror give each a reflectivity of 3% at .lambda.=3.39 .mu.m. This value is not entirely regarded as being suppressed to be enough small, and particularly for a high-power output He-Ne laser tube, it is not be allowed to neglect simultaneous laser oscillation at .lambda.=3.39 .mu.m.
Besides, the structure of the above-mentioned output mirror requires the use of three kinds of dielectric material different in refractive index and in conditions of vacuum deposition, and hence presents disadvantages: for example, much time it takes for respective vacuum evaporations and for switching work before each, associated mistakes which may occur, and correspondingly lower yield of production.